Because you promise to give it up (8)
I believe the answer is:
forswear
'up' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'because you promise to give it' is the wordplay.
I cannot quite understand how this works, but
'because' could be 'for' (I've seen this in another clue) and 'for' is found in the answer.
'promise' could be 'swear' (as in swearing something is true) and 'swear' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for forswear that I've seen before include "Retract - recant" , "Agree to do without" , "Reject or disavow" , "Deny on oath" , "Denounce" .)